A maneuvering aircraft produces seismic activity by two means. First is by rolling on the ground or runway and generating a low frequency signal, which is coupled through the earth. The second is by acoustically coupling through the air and vibrating the ground around a sensor producing a higher frequency signal. Prior art methods have not recognized the seismic energy envelopes and accordingly have not developed a method for using them for any useful purpose.
U.S. Pat. No. 3,564,493 discloses an acoustic method wherein the acoustic energy is used to distinguish between ground vehicles and aircraft. U.S. Pat. No. 4,150,568 discloses a means for measuring vibrations of a portion of a drill string by means of a transducer which produces an electrical signal representative of the vibrations. A stepping filter is used. U.S. Pat. No. 3,971,249 discloses an acoustic method for looking for a change in the operating conditions of a bearing or similar device.
U.S. Pat. No. 3,705,516 discloses a two band amplitude discrimination which measures the operating condition of a machine by measuring the ratio of predetermined portions of the vibrational output. U.S. Pat. No. 3,714,620 discloses an acoustic device which detects slower fluctuation and lower amplitude variations on some vehicle signals to distinguish between aircraft signals and land motor vehicles. U.S. Pat. No. 3,824,532 is an intrusion detection unit which discloses a system which determines instrusions in an area by the pattern of seismic disturbances and produces a signal when the seismic disturbancer passes a predetermined level.
U.S. Pat. No. 3,842,663 provides a system for locating surface defects in machinery components by utilizing the resonant frequency of a transducer which discriminates against the background vibrations of the machinery. U.S. Pat. No. 4,005,384 is an acoustic system for the firing rate of internal combustion engines. U.S. Pat. No. 3,343,167 and Pat. No. 3,362,009 requires transmitting means which sent out signals processed the returned signal.